This disclosure relates to the monitoring of motion, breathing, and heart rate of living beings, e.g., humans, in a convenient and low-cost fashion that is useful, for example, in the assessment of cardiorespiratory markers of fitness and activity, and more particularly to an apparatus, system, and method for acquiring, processing and displaying the corresponding information in an easily understandable format. In this application, reference is made to a system which can measure motion, breathing and heart rate as a cardiorespiratory monitoring device or system.
Monitoring of heart rate and respiration is of interest in assessing the performance of cardiorespiratory systems. For example, measurements of heart rate are useful when assessing fitness levels of humans, as there are well-established guidelines for physiologically normal ranges of heart rate in response to differing levels of activity. Measurements of heart rate are widely used in fitness training schedules. For example, an exercise which maintains heart rate in a range between 100 and 120 beats per minute (bpm) may be useful for fat-burning and endurance building, whereas a competitive athlete may wish to undertake activity which raises the heart rate level to 160-180 bpm. Moreover, levels have been determined which reliably adjust for age and gender, so that individuals interested in a structured cardiovascular fitness program can monitor their progress quantitatively. Accordingly, it is desirable to be able to measure heart rate in a variety of settings. However, reliable measurement of heart rate in exercise conditions poses certain technical challenges. While running or cycling, motion artifact can corrupt heart rate measurements. While swimming, electrical measurement of heart rate can be difficult due to the conducting nature of water.
In addition to heart rate, breathing rate, depth and patterns are useful indicators of the overall status of the cardiorespiratory system. It is well observed that breathing rate increases in response to exercise, but the rate of increase (or decrease during an exercise recovery period) is a marker of overall cardiorespiratory health. For persons with compromised cardiorespiratory status, who might experience dyspnoea, for example, the elevated respiratory rate is a useful marker of status.
Individual measurements of heart rate and respiration are of value, but in addition useful measurements can be derived from combinations of these measurements which provide overall markers. For example, it is known that breathing directly modulates heart rate through a physiological mechanism called respiratory sinus arrhythmia (RSA), in which the heart speeds up during inspiration, and decreases during expiration. RSA is particularly pronounced in young people, and tends to decline with age. However, in general, a high degree of RSA is associated with health, and will change in response to exercise and changes in diet (see for example, “Respiratory sinus arrhythmia alteration following training in endurance athletes,” by Ronald E. De Meersman, published in European Journal of Applied Physiology, vol. 64, no. 5, September 1992, pages 434-436). However, in order to quantify RSA, simultaneous measurements of heart rate and respiration are desirable.
Other useful parameters of cardiorespiratory fitness are the anaerobic threshold (AT) and ventilatory threshold (VT). The anaerobic threshold is the point at which the cardiorespiratory system is not providing sufficient oxygen to the muscles for the muscles' energy needs to be fully met by aerobic metabolic processes. Accordingly, the body uses its glycogen stores in an anaerobic metabolic process to maintain muscle output. At this point, the person has reached their maximum oxygen uptake, and will shortly become too fatigued to maintain their activity level (the maximum oxygen uptake is referred to as VO2,max). To measure AT accurately requires specialized laboratory equipment and blood sampling, so while this is used as a “gold standard”, it is not practical for widespread use by individuals interested in fitness. The ventilatory threshold is related physiologically to the anaerobic threshold. It is a point at which the response of minute ventilation (liters/min of air breathed) to exercise intensity becomes nonlinear, and is marked by a substantial increase in breathing rate. From an aerobic fitness point of view, it has been shown that the anaerobic threshold and the ventilatory threshold are strongly correlated. Since the goal of many fitness programs is to increase AT, it is useful to be able to use VT as a reliable surrogate marker. The cardiorespiratory monitor can be used to estimate VT by using combinations of respiration rate and heart rate. This will provide utility to the user of the monitor, as they can track the trends in their VT over long time periods (e.g., over the course of a fitness training program).
In the clinical setting, it is also useful to have reliable markers of cardiovascular fitness. For example, people suffering from heart failure have high exercise intolerance. Some subjects with heart failure are candidates for heart transplant, but given the scarcity of available hearts, doctors must prioritize patients in order of the severity of their disease. Again, for such cases, measurements of VT can be useful in assessing the overall health of the patient. A discussion of the challenges of assessing cardiorespiratory markers for assessing heart transplantation candidates is given in D. Ramos-Barbón, D. Fitchett, W. J. Gibbons, D. A. Latter, and R. D. Levy, “Maximal Exercise Testing for the Selection of Heart Transplantation Candidates—Limitation of Peak Oxygen Consumption,” Chest. 1999; 115:410-417.
A large variety of techniques exist for measurement of heart rate for the purposes of assessing cardiorespiratory fitness. Surface lead electrocardiograms (ECGs) are a highly accurate way of capturing cardiac electrical activity, and hence heart rate. However, they require that the subject attach gelled electrodes to the chest region, and also carry or wear the associated electronic processing and/or recording device. So generally, full ECG measurement is restricted to clinical applications.
More convenient techniques for electrocardiogram measurement have been introduced which trade off signal quality for convenience, and are now widely used in commercially available heart rate fitness monitors. These techniques use electrodes which are embedded in conductive textiles which are placed in proximity to the skin. Typically, the textiles form part of a chest band worn around the thorax at the level of the chest. Since the conductivity of the textile material is dependent on moisture content, these sensors work best when the person is exercising vigorously and the skin is moistened with sweat (alternatively users can apply some conducting gel to ensure good electrical measurement). The disadvantage of this system is the requirement for the person to wear the chest band, and the reduced signal quality when the person's skin is not moist.
Another technique for assessing heart rate during exercise is to use pulse oximetry, which measures the changes in reflected/transmitted light through blood vessels. A characteristic photoplethysmogram can be generated in which each cardiac contraction is visible as a distinct pulse. However, pulse oximetry methods for measuring heart rate are limited by motion artifacts and poor perfusion characteristics. The power requirements of the light emitting diodes used in oximeters can also be a limiting factor in the battery life of such a device.
Respiratory effort and breathing rate can be also measured in multiple ways. A common method for measuring respiratory effort uses inductance plethysmography, in which a person wears a tightly fitting elastic band around their thorax, whose inductance changes as the person breathes in and out. A limitation of the method from a convenience point of view is that the person has to wear a band, and remains connected to the associated electronic recording device via wires. An alternative system for measuring respiratory effort is to use impedance pneumography, in which the impedance change of the thorax is measured. The limitation of this technology is that it requires electrodes to be attached to the body, and has an active electrical component which needs to be carried by the subject.
For cardiorespiratory fitness assessment, it is also useful to measure gross bodily motion, as that is an overall indicator of daily activity and exercise intensity. The most common technique for measuring free-living activity is to use accelerometers, which can measure acceleration. When carried by a person, such devices can provide a useful indicator of the overall duration and intensity of the person's movement. such devices are often sold commercially as pedometers (step-counters). A limitation of this technology is the requirement for the person to carry the device, and the limitations of the algorithms for converting measured acceleration into activity patterns.
What is needed then, is a method, system and apparatus for measuring heart rate, respiratory rate and effort, and motion, and which overcomes various limitations of conventional approaches.